Our recent efforts have examined novel activating Ly-49 NK receptors. Murine Ly-49D augments NK cell function upon recognition of target cells expressing H-2Dd. In addition, we demonstrate that Ly-49D receptor ligation can lead to the rapid and potent secretion of IFN-"symbol"g. Cytokine secretion can be induced from Ly-49D+ NK cells after receptor ligation with Ab or after interaction with target cells expressing their H-2Dd ligand. Consistent with the dominant inhibitory function of Ly-49G, NK cells coexpressing Ly-49D and Ly-49G show a profound reduction in IFN-"symbol"g secretion after interaction with targets expressing their common ligand, H-2Dd. As an extension of these studies, we evaluated gene expression upon cross-linking the activating Ly-49D mouse NK receptor in an attempt to understand potential novel functions of these receptors in vivo. Gene expression was evaluated using a microarray from Genome Systems Incorporated (St. Louis, MO). In brief, alterations in gene expression were evaluated Mouse GEM 1 microarray chip where each chip displays a total of 8,734 elements. Although other genes were identified, the strongly induced genes fell into two categories 1) soluble factors and 2) apoptotic genes. The majority of the mRNAs that were strongly induced as analyzed by microarray hybridization were chemokine genes. Ribonuclease protection assays and chemokine protein production analysis validated the microarray results as activating the Ly-49D mouse NK receptor induced high levels of IFN-"symbol"g, lymphotactin, MIP1"symbol"a and MIP1"symbol"b mRNAs and protein following cross-linking. This gene expression was specific since other chemokines such as KC and MIP-2 were not induced. Although NK cells previously have been shown to respond to chemokines, our data demonstrates that these cells can be strong producers of these inflammatory mediators as well. Thus we conclude that a primary role for the activating NK receptors in vivo is as a trigger for soluble factor production and regulation of the immune response at the site of receptor activation. This would place NK cells and their activating Ly-49 receptors as important initiators of microbial immunity and key elements of the innate immune system.